Metal caps including caps or crowns made by shallow stamping or deep stamping of metal sheets and into which threading may be added if required, as in the case of pilfer-proof caps, include many different types. Some types are coated prior to heating while others are not. Some types have printing applied prior to heating while others may have adhesives painted on their lower inside surfaces prior to heating after which a thermoplastic resin lining is applied.
Among these metal caps, and particularly those caps used to seal the openings of containers such as bottles or wide neck jars, have been those having their inside lower surfaces lined with a packing material which has included cork or a thermoplastic resin sheet or lining material, including vinyl chloride, polyethylene or polypropylene. Use of polypropylene as a lining material has increased recently and in order to attach such as liner to the inside surface of a cap, it has required the inside lower surface of the cap to be painted with an adhesive primer after which the lining material is inserted onto the primer. Prior and after the insertion of the liner, the cap bottom is heated to about 100.degree.-200.degree. C.
A prior apparatus for the continuous heating of metal caps, and particularly crowns which have been generally made from a magnetic material such as tin plate or a tin free steel, has involved subjecting the caps to heated air. Such an apparatus is disclosed in Japanese Patent Application Publication No. 41-5588 (of 1966). In this apparatus, crowns are heated to about 160.degree. C. while being transported by a rotatable table or turntable having concave notches on the periphery engaging the face of the crowns. Such apparatus however requires an inordinate amount of time to raise the crown to the desired temperature, or if the apparatus is to perform high speed heating, such as on the order of 1,000 pieces per minute, requires a very large heating system with the result that the apparatus cannot be made compact. Further, with this apparatus, areas of the crown not requiring heating, such as the outer wall of the shell, are heated resulting in a waste of energy.
High frequency heating apparatus has been proposed for overcoming the aforementioned deficiencies and an example of such is disclosed in Japanese Patent Application Publication No. 47-41398 (of 1972). The apparatus of that application includes hairpin high frequency coils positioned beneath a conveyor on which the crowns are transported. While this apparatus can effectively heat crowns made of a strongly magnetic material where a magnetic flux can be concentrated, the apparatus is not effective for heating caps made of a non-magnetic material such as aluminum. This is because the conveyor supporting the caps has a certain thickness which is interposed between the caps and the heating coils thus reducing the mutual induction relation between the caps and the coils to a very small amount with the result that the temperature of the cap does not rise to any extent. If the cap and heating coils of this apparatus are brought close together by making the conveyor very thin in order to raise the mutual induction relation, then the caps will jump from the surface of the conveyor and tend to float above the conveyor thus lengthening the distance between the caps and the coil to reduce the induction effect and make effective heating impossible. This floating and jumping of the caps is the result of the repulsion effect occurring between the high frequency coil magnetic field and the dielectric current induced in the aluminum cap bottoms. When strongly magnetic caps such as crowns are heated in the above-described apparatus, the cap itself becomes magnetized by a magnetic field created by the high frequency current and an attraction force is formed that extends in the direction of the high frequency current conductor. As this attraction force is greater than the repulsion force due to the dielectric current, the jumping and floating described above normally will not occur.
It is an object of the present invention to overcome the difficulties associated with high speed continuous heating of non-magnetic metal caps as described above and to provide for a continuous high speed heating apparatus capable of imparting large amounts of heat to the bottoms of the metal cap.
It is a further object of the invention to provide a high frequency continuous heating apparatus which will heat the bottom parts of metal caps uniformally.